Multiple Input/Multiple Output (MIMO) is a technique used in wireless communications in which multiple transmitting and receiving antennas are used to improve communication quality. Transmission precoding is a technique to improve performance of MIMO systems, and reduce the complexity of MIMO receivers. Transmission precoding may require receiving feedback.
To reduce feedback overhead, a predetermined codebook may be used for precoding so that only an index to the codebook is needed for feedback. A transmitting unit uses a precoding matrix index (PMI) sent from a receiving unit to precode transmitted data. For data detection at the receiving unit, the same precoding matrix is required. If the transmitting unit and the receiving unit end up using different precoding matrices due to feedback error, the error rate for data detection will rise.
To avoid a precoding mismatch between the transmitting and receiving units, the transmitting unit often precodes known symbols in the same manner as data and transmits the precoded symbols. Such known symbols are referred to as dedicated pilot symbols. These symbols may then be used by a receiving unit to estimate the actual precoding matrix used in the data packet that has been received. This process is known as precoding validation.